1. Field of the Invention
The present invention relates to an antenna apparatus, and more particularly to an antenna apparatus capable of directivity control.
2. Description of the Background Art
FIG. 23 is a plan view showing a main part of a conventional antenna apparatus capable of directivity control. In FIG. 23, this antenna apparatus includes four antenna elements 51-54 successively arranged at regular angular intervals around the central point on the same plane. Antenna elements 51-54 respectively have feed points P1-P4. Antenna elements 51 and 53 and antenna elements 52 and 54 respectively form dipole antennas. The antenna apparatus is used to receive electric waves for a plurality of UHF (Ultra High Frequency) television broadcastings. Antenna elements 51-54 are arranged horizontally, and for example, antenna element 52 is oriented in the direction of a main UHF television broadcast station. It is assumed that the direction of antenna element 52 is 0 degree as viewed from the central point. The directivity of the antenna apparatus is controlled in the direction of the UHF television broadcast station selected from a plurality of UHF television broadcast stations by a viewer.
FIGS. 24A-24D are diagrams showing a directivity control method of this antenna apparatus. As shown in FIG. 24A, when the 8-shaped directivity of the antenna apparatus is oriented in the 0 degree direction (forward) and the 180 degree direction (backward), feed points P1, P3 are connected to a television receiver. An electric wave transmitted from the 0 degree direction is received by antenna elements 51, 53 and applied to the television receiver through feed points P1, P3. On the other hand, an electric wave transmitted from the 180 degree direction is received by antenna elements 51, 53 and applied to the television receiver through feed points P1, P3. Therefore, the directivity of the antenna apparatus is 8-shaped directivity in which reception sensitivity is maximum in the 0 degree direction and the 180 degree direction, as shown by a dotted line in FIG. 24A.
Furthermore, as shown in FIG. 24B, when the 8-shaped directivity of the antenna apparatus is oriented in the 90 degree direction (horizontal right direction) and the 270 degree direction (horizontal left direction), feed points P2, P4 are connected to the television receiver. An electric wave transmitted from the 90 degree direction is received by antenna elements 52, 54 and applied to the television receiver through feed points P2, P4. On the other hand, an electric wave transmitted from the 270 degree direction is received by antenna elements 52, 54 and applied to the television receiver through feed points P2, P4. Therefore, the directivity of the antenna apparatus is 8-shaped directivity in which reception sensitivity is maximum in the 90 degree direction and the 270 degree direction, as shown by a dotted line in FIG. 24B.
Furthermore, as shown in FIG. 24C, when the 8-shaped directivity of the antenna apparatus is oriented in the 45 degree direction (diagonally forward right direction) and the 225 degree direction (diagonally backward left direction), transmission lines 55, 56 are respectively connected between feed points P1 and P2 and between feed points P3 and P4, and feed points P1, P4 are connected to the television receiver. Thus, antenna elements 51 and 52 form one antenna element while antenna elements 53 and 54 form another antenna element, and the formed two antenna elements receive electric waves from the 45 degree direction and the 225 degree direction. Therefore, the directivity of the antenna apparatus is 8-shaped directivity in which reception sensitivity is maximum in the 45 degree direction and the 225 degree direction, as shown by a dotted line in FIG. 24C.
In addition, as shown in FIG. 24D, when the 8-shaped directivity of the antenna apparatus is oriented in the 135 degree direction (diagonally backward right direction) and the 315 degree direction (diagonally forward left direction), transmission lines 55, 56 are respectively connected between feed points P1 and P4 and between feed points P2 and P3, and feed points P1, P2 are connected to the television receiver. Thus, antenna elements 51 and 54 form one antenna element while antenna elements 52 and 53 form another antenna element, and the formed two antenna elements receive electric waves from the 135 degree direction and the 315 degree direction. Therefore, the directivity of the antenna apparatus is 8-shaped directivity in which reception sensitivity is maximum in the 135 degree direction and the 315 degree direction, as shown by a dotted line in FIG. 24D.
FIG. 25 is a diagram showing the frequency characteristic of the gain of this antenna apparatus. In FIG. 25, the range of frequency is 470-806 MHz, which range includes the frequency ranges of UHF television broadcasting in Japan and the United States. In Japan, the frequency range of broadcasting electric waves in UHF television broadcasting is 470-770 MHz (13-62 channels). In particular, in the case of terrestrial digital broadcasting, the frequency range is 470-710 MHz. On the other hand, in the case of UHF television broadcasting in the United States, the frequency range is 470-806 MHz. As can be understood from FIG. 25, this antenna apparatus has sufficiently high gain in 470-806 MHz.
FIGS. 26A-26F are diagrams showing the frequency characteristics of 8-shaped directivity in the 0 degree direction of this antenna apparatus. As can be understood from FIGS. 26A-26F, this antenna apparatus has good 8-shaped directivity in 470-806 MHz.
There also exists an antenna apparatus which realizes 8-shaped directivity in four directions by combining the directivity of two dipole antennas arranged orthogonal to each other (see, for example, Japanese Patent Laying-Open No. 2006-157209).
The conventional antenna apparatus is, however, susceptible to multipath interference since it has 8-shaped directivity. For example, since the reception sensitivity for electric waves from the front and the reception sensitivity for electric waves from the back are the same, if a building stands at the back of the antenna apparatus, the electric wave from the front and the electric wave reflected on the building at the back are combined, resulting in unclear images.